The present invention relates in general to well logging tools and in particular to well logging tools for measuring formation resistivity.
Induction well logging tools were originally designed to provide a resistivity measurement in wells drilled with electrically insulating, oil-based mud. Since their introduction, their use has spread to wells drilled with water-based muds of moderate-to-high electrical conductivities. The basic element in all multi-coil induction tools is the two-coil sonde. The two-coil sonde consists of a single transmitter coil and a single receiver coil wrapped around an insulating mandrel. The transmitter coil is driven by an oscillating current at a frequency of a few tens of kilohertz. The resulting magnetic field induces eddy currents in the formation which are coaxial with the tool. These eddy currents produce a magnetic field which in turn induces a voltage in the receiver coil. This induced voltage is then amplified, and the component of the voltage that is in-phase with the transmitter current is measured and multiplied by a tool constant to yield an apparent conductivity signal. This apparent conductivity is then recorded at the surface as a function of the depth of the tool.
The two-coil sonde has several practical limitations. Its response is adversely affected by several factors including the borehole, adjacent beds, and mud filtrate invasion. Also, the two-coil sonde is difficult to implement because of the large direct mutual coupling between the coils. Even though this mutual signal is out of-phase with the transmitter current, it is a problem because a very small phase shift in the electronics can cause this mutual coupled signal to "leak" into the apparent conductivity signal. For these reasons, it is the standard practice in the industry to construct induction logging tools with coil arrays which include additional coils. Typically, there are several transmitter coils and several receiver coils. In certain applications all of the transmitter coils may be connected in series into one circuit. Similarly, all of the receiver coils may be connected in series in a separate circuit. The additional coils served to cancel out the various adverse effects listed above. Such arrays are generally termed "focused arrays".
The following are terms of art that are used often to compare various induction tools. The "vertical resolution" of a tool is a measure of the thinnest bed that a tool can detect. That is, a tool may accurately estimate the thickness of beds that are thicker than it's "vertical resolution". A tool can also accurately locate a bed boundary to within the tolerance of its "vertical resolution". There is still a significant error in the apparent conductivity reading in a thin bed due to signals from adjacent beds; however, so long as the thin bed is thicker than the vertical resolution of the tool, the tool can estimate the thickness of the bed. This error in the apparent conductivity reading of a thin bed due to the signal coming from adjacent beds is referred to as "shoulder effect." In known induction tool arrays, the additional coils are arranged to cancel out much of this shoulder effect.
It is also possible for a tool to have good vertical resolution but poor shoulder effect. Such a tool would be able to accurately define bed boundaries but would give poor estimates of the conductivities of these thin beds. Vertical resolution and shoulder effect are two aspects of the vertical focusing of an induction tool coil array.
The "depth of investigation" of a tool is a measure of how deeply the tool sees into the formation. The "depth of investigation" is defined as the radius of the cylinder from which half the apparent conductivity signal comes. The "borehole effect" is a measure of how much signal comes from the borehole as compared to the formation. In conventional arrays, coils are arranged to cancel much of the signal coming from near the tool so that the "depth of investigation" will be large and the "borehole effect" will be small.
The foregoing discussion can be understood by assuming that a tool may be operated at a sufficiently low frequency so that there is no significant attenuation of the transmitted signal as the signal propagates through the formation. In practice, such attenuation cannot be neglected since it reduces the transmitted signal proportionately more in conductive formations. This error is commonly referred to as "skin effect." Prior art practitioners generally attempt to design a coil array which has moderate skin effect at the highest conductivity of interest in logging situations and then correct for the skin effect at the surface. The skin effect correction is typically a correction which yields the true conductivity of a homogeneous formation.
In the case of conventional induction tool arrays, coils must be positioned to define the tool's vertical resolution, depth of investigation, as well as to compensate for borehole and shoulder effect. In addition, the coils must minimize the mutual coupling between transmitter coils and receiver coils, as this signal is very large when compared to most formation signals. In known coil arrays, the position and strength of each coil controls each of these aforementioned effects. Because each of these effects may change as a coil is modified, it is difficult to design a coil array optimized to reduce all of these effects simultaneously. The different effects interact, as one effect is reduced, another is increased. Conventional coil array designs therefore must be a compromise between sharp vertical resolution and deep radial penetration into the formation.
It would be desirable for an induction logging tool to permit variation in depth of investigation or vertical resolution independently without compromising the other parameter. Previously, such a tool has not been available.
In most commercial applications, it is also desired to investigate the strata surrounding a borehole to different depths of investigation in order to determine the diameter of invasion of the strata by borehole fluids. This requires at least two measurements with contrasting radial response and ideally identical vertical resolution so that differences in the logs obtained will be due to radial anomalies in the formations such as invasion.
Most prior art dual induction tools utilize deconvolution filters to match dissimilar vertical responses of two induction coil arrays with inherently different vertical resolutions by smoothing out the response of the array with the sharper vertical resolution and degrading it to match the vertical response of the second array. This approach is not desirable in view of the degradation of vertical resolution which is necessary to match the coil arrays utilized to investigate the formation.
Thus, a need exists for a dual induction tool capable of investigating multiple depths of investigation while maintaining substantially identically vertical resolution for all coil arrays.